Study on the Formation Mechanism and Microphysical Characteristics of Warm-Sector Convective System with Multiple-Rain-Bands Organizational Mode

Based on ERA5 reanalysis data and multi-source observations, including polarimetric radar and automatic weather stations, this study analyzes the formation mechanism and microphysical characteristics of a warm-sector heavy rainfall event caused by a convective system with multiple-rain-bands organizational mode over the western coast of south China. In the early stage, under the influence of coastal convergence and topography, convection was triggered in the coastal mountainous areas and moved north-eastwards. Nocturnal cooling induced the north winds in the inland mountainous area. A mesoscale convergence line was formed in the middle of Yangjiang city between the inland north and coastal south winds, which facilitated the developing and merging of convective storms into a linear convective band along the convergence line. This relatively long convective band presented a quasi-stationary state in the south of Mt. Ehuangzhang and Mt. Tianlu, which results in the first precipitation peak. At this stage, the convection developed to a higher level, with relatively larger raindrops, producing larger amounts of rainfall, which was probably related to the active merging of convection. In the later phase, as the environmental winds shifted, convective bands tended to move southeastwards, accompanied with the cold pools. At the same time, the multiple short convective bands were formed, which were almost parallel to the shear line, and a multiple-rain-bands organizational mode occurred. The mesoscale convergence line maintained due to the outflows of cold pools caused by precipitation in the preceding period, and then gradually moved southwards. Under the influence of the mesoscale convergence and topography, convection was continuously triggered at the southern end of the short convective bands. This back-building characteristic favored the development of the convective system. The multiple rain bands passed through the same place in a"rainband-training" form, resulting in the second peak of precipitation. The collision process was active in the low levels during this event.